patch 8.2.1923: Vim9: "filter" command modifier doesn't work

Problem:    Vim9: "filter" command modifier doesn't work.
Solution:   Check for space on char before argument. (closes #7216,
            closes #7222)

src/testdir/test_vim9_cmd.vim

@@ -312,6 +312,18 @@ def Test_filter_is_not_modifier()
   assert_equal([#{x: 3, y: 4}], tags)
 enddef
 
+def Test_filter_is_recognized()
+  var lines =<< trim END
+    final expected = "\nType Name Content\n  c  \"c   piyo"
+    @a = 'hoge'
+    @b = 'fuga'
+    @c = 'piyo'
+
+    assert_equal(execute('filter /piyo/ registers abc'), expected)
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_eval_command()
   var from = 3
   var to = 5